Stacked series regulator



Dec. 3, 1968 T. G. HARRIGAN ETAL STACKED SER I ES REGULATOR 2Sheets-Sheet 1 Filed April 18, 1966 LOAD . 3 a I F 68 VOLTAGE ACROSSGATTERV 3/ 7.'G.HARRIGAN mvavrops W JEWETT ZZM A TTORNEV, v

T. G. HARRIGAN ETAL 3,414,802

STACKED SERIES REGULATOR Dec. 3, 1968 2 Sheets-Sheet 2 Filed April 18,1966 Fla; 4

FIG. 5

a w o L R R R 0 m m N A A4 mm mm as m m m n R P l 9 w w r U U U CD, P pp P P 1 P l DU D U D U 5 5 s h United States Patent "ice 3,414,802STACKED SERIES REGULATOR Thomas G. Harrigan, Lake Hiawatha, and WilliamE. Jewett, Basking Ridge, N.J., assignors to Bell Telephone LaboratoriesIncorporated, New York, N.Y., a corporation of New York Filed Apr. 18,1966, Ser. No. 543,319 6 Claims. (Cl. 32116) ABSTRACT OF THE DISCLOSUREAn eflicient votlage regulating system comprises two variable seriesimpedance regulators with outputs in parallel. The first regulatoroperates efficiently with a minimum voltage drop across it when theinput voltage is normal. The second regulator, operating from a higherinput voltage, is adjusted to supply a slightly lower output voltagethan the first regulator supplies. The second regulator is thereforenormally cut-01f and operates only when the output voltage falls becauseof a larger drop in the input voltage than the first regulator canabsorb. Where a slight step in load voltage is undesirable the output ofthe second regulator is connected to the first regulators input. Wherelarge excursions in input voltage are bidirectional three regulators areconnected with their outputs in parallel.

This invention relates to the field of voltage regulated power supplies,particularly to the series regulating types. In the electronic artsdirect current is commonly supplied at a constant regulated voltage to avariable load from an unregulated source through a series regulator ofeither the switching or the nonswitching type.

In the switching type regulator a switch is interposed between thesource and the load. The percentage of time that the switch is closed,transmitting current to the load, is varied to maintain a constantvoltage at the load. The switch must operate, of course, at a fastenough rate to accommodate contemplated changes in the load and in thesource. When the switch is closed there is virtually no voltage dropacross it; when the switch is open no current flows through it.Consequently, there is virtually no power lost in the switch itself, andthe device is very efficient. On the other hand, since the current issupplied in pulses, extensive filtering is required both at the inputand the output, and the output impedance is relatively high. Inaddition, the size and weight of this type regulator tend to berelatively large.

In the nonswitching or variable impedance type series regulator animpedance element such as a transistor or vacuum tube is interposedbetween the source and the load. The impedance of the element is variedto maintain a constant voltage at the load. Since the response of vacuumtubes and transistors is virtually instantaneous, very little filteringis needed. In fact, the device itself is a type of electronic filter. Inaddition, the size and Weight can be made relatively small. The majordrawback, however, is poor efficiency. All of the load current passesthrough the impedance element, and all of the voltage difference betweenthe highest input voltage and the lowest output voltage is droppedacross it. Under the common condition of a high input voltage and a lowvoltage high current load an appreciable voltage appears across theregulator at a high current. This results in a high power loss and verypoor efiiciency.

The prior art approaches a solution to the inelficiency of the otherwisedesirable series nonswitching regulator in two ways. First, when a largevariation in output voltage is desired, means have been used forlowering the source voltage as the load voltage is lowered. Commonly avariable autotransformer at the input is coupled me- 3,414,802 PatentedDec. 3, 1968 chanically to the potentiometer which adjusts the outputvoltage. The second common approach attacks an effect of the poorelficiency rather than the cause. In the case of a transistor regulatoran instantaneous large power absorption can result in permanent damageto the transistor. Many devices have been designed which protect thetransistor by absorbing this power elsewhere, but these, of course, donot improve the efficiency.

In applications where the source voltage is relatively constant withonly an occasional large excursion and where the equipment is inconstant use, there is a real need for a series regulator which canoperate efficiently and yet accommodate the occasional large inputexcursion. Such an application is commonly found in a telephone system.In order to provide continuous service in the event of a power blackout,the system continually operates on storage batteries under a tricklecharge; normally the voltage at the batteries is relatively constant.When there is a power failure, however, the charging voltage disappearsand the battery voltage drops. An efiicient regulator is found in ourinvention, the stacked series regulator.

An object of this invention is to provide a very efficient voltageregulating circuit with all the advantages of series nonswitchingregulation and without the major disadvantage of low efliciency.

Another object is to provide a series voltage regulaing circuit ideallysuited to supply current from a normally well regulated source such asstorage batteries.

A still further object of this invention is to provide a series voltageregulating circuit in applications where generated heat or cost ofwasted power must be kept to a minimum.

In its simplest form the invention comprises two ordinary seriesregulators operating with outputs in parallel. The first regulator isdesigned to operate with a minimum voltage across it when the inputvoltage is normal. This allows it to operate Very etficiently. Thesecond regulator, operating from a higher input voltage, is adjusted tosupply current at a slightly lower output voltage than the firstregulator supplies. This second regulator, because of its higher drop,is not so eflicient when operating, but is normally cut off. During theinfrequent periods when the input voltage to the first regulator dropsbelow the value at which the regulator can continue to regulate, theoutput voltage drops slightly causing the second regulator to becomeoperative. Thus the system operates with good efliciency most of thetime, protects for the occasional extreme excursion, and has all theadvantages of series nonswitching regulation.

In the event that the slight step in output voltage necessary toactivate the normally cut off regulator is undesirable, a similarembodiment can be used. In this embodiment the output of the secondregulator is connected to the first regulators input rather than itsoutput. Now, when the first source voltage drops to some predeterminedpoint above that at which the first regulator loses regulation, thesecond regulator begins to pass current, maintaining the input voltageto the first regulator. The system output voltage therefore remainssubstantially constant, even during the large input excursion.

Where the normal regulation of the first source is within the tolerancerequired by the load, the first regulator may be eliminated entirely. Inthis case, during normal operation, the load is supplied directly fromthe first source with no power consumed in a regulator. When the loadvoltage, which is the first source voltage, drops beyond the normalminimum, the second regulator takes over and maintains the load voltage.

Finally, where the input voltage is apt to rise considerably as well asfall, still another embodiment of our invention utilizes a thirdregulator to handle the large positive excursion. This regulator isadjusted to regulate at a slightly higher output voltage than the firstregulator and is supplied from a lower voltage DC source. All three DCsources are supplied from a single system input. During normaloperation, the third regulator is cut off because its source voltage islower than the load voltage. During a large positive voltage excursionon the system input, the source voltage to the third regulator rises,allowing that regulator to pass current and take over the regulation atthe slightly higher load voltage.

A more complete understanding of the invention may be obtainedtrom astudy of the following detailed description of several specificembodiments. In the drawmgs:

FIG. 1 is a block diagram of the invention in its simplest form;

FIG. 2 is a schematic diagram of one preferred embodiment of theinvention;

FIG. 3 is a plot of output voltage against input voltage for two typicalloads on the system of FIG. 2;

FIG. 4 is a block diagram of an alternative embodiment of the inventionwith a different arrangement of regulators; and

FIG. 5 is a block diagram of an alternative embodiment utilizing threeregulators.

In the embodiment of the invention shown in FIG. 1, a DC source 1.supplies current to a load 2 through a series regulator 3 via a pair ofconductors 5 and 6. In like manner, a DC source 7 can supply current toload 2 through a series regulator 8 via a pair of conductors 9 and 10.Source 7 is of higher voltage than source 1 and regulator 8 is set tosupply current at a slightly lower output voltage than is regulator 3.Normally, therefore, when source 1 is operating close to its nominalvoltage, virtually all of the current to load 2 is supplied by source 1through regulator 3. Regulator 8, in attempting to reduce the loadvoltage, has a very high impedance, cutting off the current from source7.

When the voltage of source 1 falls oil, the impedance of regulator 3decreases in an attempt to maintain the voltage at the load. When thisimpedance is at a minimum and the voltage of source 1 continues to fall,the voltage at the load must then fall. When the load voltage, which isthe output voltage of regulator 8 as well as regulator 3, falls a slightamount to that value to which regulator 8 was set, the impedance ofregulator 8 begins to fall. The load voltage is then maintained at theslightly lower level by current from source 7 until the voltage ofsource 1 again rises.

In the preferred embodiment of FIG. 2, an output tap 21 of a DC to DCconverter 30 supplies current to a load 22 through a series regulator 23via a pair of conductors 25 and 26. In like manner, an output tap 27 cansupply current to load 22 through a series regulator 28 via a conductor29 and conductor 26. Tap 27 supplies a higher voltage than does tap 21.Converter 30 is supplied by a battery 31 which is kept under constantcharge by a charger 32 operating from the AC power line.

Series regulators 23 and 28 are typical of those well known in the artand their operation will be described. The invention is not limited tothe use of these particular regulators, of course, as it will operateequally well with any DC series voltage regulator circuit. Apotentiometer 35 is in parallel with the load 22 hence sees the samevoltage. A series circuit consisting of a Zener diode 36 and a resistor37 is also in parallel with the load. The Zener diode is poled so thatit is back biased by the load voltage and the value of resistor 37 ischosen so that avalanche current continually flows in the diode. Underthese conditions, the voltage across the diode is constant, independentof changes in the voltage across the load 22. The emitter 38 of an NPNtransistor 39 is connected to the junction between Zener diode 36 andresistor 37. The base 40 of transistor 39 is connected to the adjustabletap 41 of potentiometer 35. Any change of output voltage across load 22therefore appears undiminished at emitter 38 and some proportion of thischange appears at base 40. It because of a change in current in load 22or a change in voltage across battery 31 the voltage across load 22tends to drop, the voltage at emitter 38 drops the same amount whilethat at base 40 drops a lesser amount. This causes a slight increase inbase-emitter voltage of transistor 39 and hence a slight increase inbase-emitter current. The collector 43 of transistor 39 is connected tothe base 44 of a PNP transistor 45. The collector 46 of transistor isconnected to conductor 25; the emitter 47 is connected to the base 48 ofa PNP transistor 50. The emitter 51 of transistor is connected to tap21; the collector 52 is connected to conductor 25. Biasing resistor 53is connected between tap 21 and base 44, and biasing resistor 54 isconnected between tap 21 and base 40.

The slight increase in base-emitter current of transistor 39 causes bytransistor action a larger increase in collector-emitter current. Inlike manner this causes a larger increase in collector-emitter currentin transistor 45 and in turn a still larger increase incollector-emitter current in transistor 50. The collector-emitterimpedance of transistor 50 is thereby reduced, offsetting the originaldrop in output voltage at load 22. Should the output voltage tend torise, the impedance of transistor 50 is increased through a similarchain of events of opposite polarity. The output voltage at load 22 istherefore well regulated over a range of input and output conditions.

This invention departs from the prior art, however, in that the range ofinput voltage over which regulator 23 maintains the output voltage ispurposely made very small. It is important to the eificiency objectiveof the invention that the voltage of tap 21 be so chosen in relation tothe desired output voltage that the voltage drop across transistor 50 benever very large. In fact, the collector-emitter impedance of transistor50 is at an effective minimum when the voltage of battery 31 is at itsnormal minimum with battery charger 32 operating. This condition can bebetter explained with additional reference to the curves of outputvoltage against input voltage of FIG. 3.

Curve 61 is for a higher current load; curve 62 is for a lower currentload. For example, curve 61 may represent five amperes, while curve 62represents one ampere. Both scales have been considerably amplified toillustrate the action. The upper portion of curve 61, above point 64,represents the condition where regulator 23 is functioning as described.The voltage across load 22 is maintained relatively constant as long asthe voltage at battery 31 does not go below the value represented byabscissa 65. This value should be by design the minimum normallyoccurring voltage across battery 31 with the charger operating. At thispoint also regulator 28 is not supplying any current. For sake ofillustration, regulator 28 is similar to regulator 23. Threetransistors, 70, 71 and 72 are connected similarly to transistors 50, 45and 39, respectively. A potentiometer 73 for adjusting output voltagecorresponds to potentiometer 35; a voltage reference circuit comprisinga Zener diode 74 and a resistor 75 corresponds to the circuit of Zenerdiode 36 and resistor 37; a bias resistor 76 corresponds to resistor 53.Transistors 70, 71 and 72 are all cut off by virtue of the setting ofpotentiometer 73, which controls the bias on transistor 72. Hence duringover 99.9 percent of the time, when the power lines are intact, theefiiciency of the system is very good.

Now should the DC input voltage across battery 31 continue to drop belowabscissa in FIG. 3, as in the instance of a line power failure whichmakes charger 32 inoperative, output voltage begins to drop because theimpedance of transistor 50 can go no lower. When the output voltagedrops to point 66, however, represented by ordinate 67, the emittervoltage of transistor 72 is low enough to turn on transistor 72, in turnturning on transistors 71 and 70, and regulator 28 begins to functionmuch as did regulator 23. A further drop in battery voltage haspractically no effect on output voltage because of the action ofregulator 28. The difference between point 64 and point 66 can be madevery small, in the order of one-tenth volt compared to an output oftwenty-four volts. Contrary to regulator 23, regulator 28 may bedesigned with a large reserve voltage. That is, the voltage at tap 27may be considerably higher than the desired load voltage. While thisproduces inefficiency, the effect on overall efficiency is slightbecause regulator 28 operates for such a small percentage of time.

Even in the worst condition, however, when regulator 28 is supplying allof the load current, the system is more efiicient than would be a singleseries regulator under normal conditions. This is because during thisfault condition the battery voltage, hence the voltage at tap 27, and inturn the voltage across the functioning regulator 28 are below normal.

As can be seen by curve 62, at a lighter load the crossover point fromregulator 23 to regulator 28 takes place at a lower input voltage,abscissa 68. A capacitor 55 across the load is useful to reduce noisevoltage.

Where the step in load voltage shown in FIG. 3 is undesirable, thecircuit of FIG. 4 may be used. By way of illustration, a source 130 maycomprise the multitapped secondary winding 131 of a transformer or DC toDC converter, with a rectifier 132 connected to each tap except thecenter tap. The cathodes of the rectifiers of symmetrical taps areconnected together to provide fullwave rectified direct current atterminals 127 and 121 with the common ground terminal 133. Thisarrangement may be used for the sources of any of the embodiments of theinvention. A series regulator 123 is supplied from terminals 121 and 133over conductors 125 and 126. A load 122 is in turn supplied fromregulator 123. As in the case of regulator 3 of FIG. 1 and regulator 23of FIG. 2, regulator 123 is designed with .a small reserve voltag Thatis, if the voltage at the terminals 121 and 133 an hence at the input ofregulator 123, points 135 and 136, were to fall a small amount, below aconvenient infrequent minimum, the load voltage would start to drop. Theoutput of a series variable impedance regulator 128, however, isconnected to the input of regulator 123. Terminals 127 and 133 areconnected to the input of regulator 128. Regulator 128 is adjusted tosupply current at a voltage less than the normal voltage appearing attap 127, but higher than that at which regulator 123 ceases to regulate.Most of the time, therefore, when the voltage at terminal 121 is withinits normal limits, regulator 128 is cut off, and current is supplied toload 122 through efiicient regulator 123. When the voltage at terminal121, which is the output voltage of regulator 128, begins to drop beyondits normal limits, however, the impedance of regulator 128 begins todrop, passing current from terminal 127 and maintaining the voltage atpoints 135 and 136 above that which produces a drop at load 122.

In the event that the load can tolerate the normal voltage range of theprimary source 121, and must be protected from only a major excursion,it is possible to eliminate regulator 123 entirely as shown by thedotted lines in FIG. 4. This is the most efficient embodiment of theinvention in that there is no power lost in regulation during normaloperation. There is no change in the operation of regulator 128.

For applications where the infrequent large excursions in input voltagetend to be bidirectional, a third regulator may be added, as shown inblock diagram FIG. 5. In this circuit, for sake of illustration, threeDC power supplies 91, 92 and 93 are each powered by a common AC inputline 94. The output circuits of the three supplies are connected inseries. A load 82 is connected across the parallel connected outputcircuits of three regulators 81, 83 and 84. The input circuit ofregulator 83 is connected between the positive output terminal of supply91 and the negative output terminal of supply 93; the input circuit ofregulator 81 is connected between the positive output terminal of supply92 and the negative output terminal of supply 93; the input circuit ofregulator 84 is connected between the positive and negative outputterminals of supply 93. The invention is not limited, of course to thissupply voltage polarity, as the opposite polarity would work equallywell. During the preponderance of time, when the AC input voltage iswithin normal limits, the efficient regulator 81 supplies current toload 82 from the source made up of supply 93 in series with supply 92.Similarly to previously discussed circuits, regulator 83 is adjusted tosupply current at a slightly lower voltage than is regulator 81. Whenthe AC input voltage drops beyond the regulating range of regulator 81,regulator 83 becomes active and supplies current to load 82 from thesource made up of supplies 91, 92 and 93 in series. Regulator 84 isadjusted to supply current at a slightly higher voltage than isregulator 81. Consequently, the true impedance of regulator 84 is at aminimum as long as the load voltage is normal or lower. The only reasonthat regulator 84 does not supply current is that DC supply 93 is not ofa high enough voltage. When the AC input voltage in line 94 risesappreciably, therefore, the voltage at DC supply 93 rises, allowingregulator 84 to take over and supply the load.

It is to be understood that the above-described arrangements areillustrative of the applications of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention,

What is claimed is:

1. A direct current voltage regulating system comprising a first directcurrent variable impedance type series voltage regulator having inputand output terminals, a first direct current source connected to saidinput terminals, a second direct current variable impedance type seriesvoltage regulator having input and output terminals, and a second directcurrent source connected to the input terminals of said secondregulator, wherein the output terminals of both regulators are connectedin parallel, said second direct current source is of higher voltage thansaid first source, and said second regulator provides current at aslightly lower output voltage than said first regulator.

2. A direct current voltage regulating system as in claim 1 wherein saidsources comprise a direct current to direct current converter having aninput circuit and multiple output voltage taps, a direct current storagebattery connected to said converter input circuit, and charging meansconnected to said battery for maintaining the voltage thereof.

3. A direct current voltage regulating system as in claim 1 comprisingin addition a third direct current series voltage regulator having inputand output terminals and a third direct current voltage source connectedto the input terminals of said third regulator, wherein the outputterminals of said third regulator are connected in parallel with thoseof said first and second regulators, and said third regulator providescurrent at a slightly higher voltage than does said first regulator.

4. A direct current voltage regulating system as in claim 1 wherein saidfirst and second direct current series voltage regulators each compriseinput and output terminals, adjustable sensing means connected acrosssaid output terminals for sensing a portion of the voltage thereappearing, voltage reference means, comparison means connected betweensaid sensing means and said reference means for producing an outputvoltage proportional to the difference between said sensing means andsaid reference means, and current control means connected between oneinput and one output terminal responsive to the output voltage of saidcomparison means.

5. A direct current voltage regulating system comprising a firstvariable impedance type series regulator having input and outputterminals and adapted to maintain a constant voltage at said outputterminals, a load connected to said output terminals, a first directcurrent source having a nominal voltage range connected to said inputterminals, a second variable impedance type series regulator havinginput and output terminals and adapted to maintain an adjustableconstant voltage at its output terminals, a second direct current sourceconnected to said last-named input terminals, and means connecting theoutput terminals of said second regulator to the input terminals of saidfirst regulator, wherein the voltage magnitude of said second source isgreater than that of said first source and said second regulatorsupplies current at a lower voltage magnitude than said nominal voltagerange.

6. A direct current voltage regulating system as in claim 5 wherein saidfirst and second source comprises the rectified outputs of a multitaptransformer.

References Cited UNITED STATES PATENTS 2,701,858 2/1955 Bakeman et a1.32116 3,001,082 9/1961 Clarke 30753 X 3,135,910 6/1964 Hamilton 321163,161,778 12/1964 Harrison et al. 307-61 3,356,855 12/1967 Suzuki et a1.30753 LEE T, HIX, Primary Examiner.

A. D. PELLINEN, Assistant Examiner.

